Understanding the mechanisms by which the environment affects the development of reading-related cognitive skills and reading outcome has important implications for education and the prevention of reading problems. Although informative, many environmental studies of reading have not taken into account the potential mediating effects of genes upon reading-related environmental influences. The long-term goal of this study therefore is to systematically examine the effect of genes upon reading-related environmental influences. The long-term goal of this study therefore is to systematically examine the effect of environmental influences upon the acquisition of reading, using a genetically-sensitive research design. The proposed research has four specific aims: (1) To examine genetic and environmental influences on the growth of reading and related cognitive and behavioral skills, (2) To examine the correlation between distal and proximal environmental factors and the growth of reading and related cognitive and behavioral cognitive and behavioral skills, and (4) To examine the links between sibling differences in environments and sibling differences in the growth of reading and related cognitive and behavioral skills. A longitudinal twin study is proposed to achieve these aims. A sample of 350 MZ and same-sex DZ twin pairs will be assessed at 5.5, 6.5, and 7.5 years of age using a comprehensive battery of cognitive, reading-related, and environmental measures. Additionally, parents and teachers of the twins will be asked to provide additional information concerning each twin's cognitive and reading level at each point of assessment, as well aspects of each twin's environment. The proposed study is innovative in its integration of environmental theory, empirical research examining the underpinings and development of reading outcome, and behavioral genetic methodology. Practically, developing a more informative model of how genes and environments work together to yield reading outcome has important implications on how to prevent, detect, and ameliorate reading problems.